Ethylene hydroformylation and carbon monoxide hydrogenation over modified and unmodified silica supported rhodium catalysts

Ethylene hydroformylation and carbon monoxide hydrogenation (leading to met hanol and C-2-oxygenates) over Rh/SiO2 catalysts share several important co mmon mechanistic features, namely, CO insertion and metal-carbon (acyl or a lkyl) bond hydrogenation. However, these processes are differentiated in th at the CO hydrogenation also requires an initial CO dissociation before cat alysis can proceed. In this study, the catalytic response to changes in par ticle size and to the addition of metal additives was studied to elucidate the differences in the two processes. In the hydroformylation process, both hydroformylation and hydrogenation of ethylene occurred concurrently. The desirable hydroformylation was enhanced over fine Rh particles with maximum activity observed at a particle diameter of 3.5 nm and hydrogenation was f avored over large particles. CO hydrogenation was favored by larger particl es. These results suggest that hydroformylation occurs at the edge and corn er Rh sites, but that the key step in CO hydrogenation is different from th at in hydroformylation and occurs on the surface. The addition of group II- VIII metal oxides, such as MoO3, Sc2O3, TiO2, V2O5, and Mn2O3, which are ex pected to enhance CO dissociation, leads to increased rates in CO hydrogena tion, but only served to slow the hydroformylation process slightly without any effect on the selectivity. Similar comparisons using basic metals, suc h as the alkali and alkaline earths, which should enhance selectivity for i nsertion of CO over hydrogenation, increased the selectivity for the hydrof ormylation over hydrogenation as expected, although catalytic activity was reduced. Similarly, the selectivity toward organic oxygenates (a reflection of the degree of CO insertion) in CO hydrogenation was also increased. (C) 2000 Elsevier Science B.V. All rights reserved.